Apart from
homeotic and floral organ identity genes, transcription factors also plays an
important role in sex deteremination in plants. CUC2, a NAC transcription factor was expressed at boundaries
between meristems and organ primordia and plays an important role in organ
separation and female reproductive organs in Arabidopsis thaliana
and Silene latifolia 32. bZIP (homolog of PERIANTHIA (PAN)) was involved in flower
development by altering floral organ number and initiation pattern by
activation of the C-class MADS box protein AG in A. thaliana (Running and Meyerowitz, 1996; Wynn et al., 2014; Das et al., 2009; Maier et al.,2009). SUP a C2H2 type zinc finger protein is a negative regulator of B class of genes. CEN1, TFL1 and FT belongs to the TCP (TEOSINTE BRANCHED1, CYCLOIDEA, PCF) family of
transcription factors. TFL1 and FT have
antagonist effects where FT promotes flowering and TFL1 is a suppressor of
flowering.

2.3.5 Role of hormones in sex
determination

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Flower
development and sex determination is highly regulated by crosslink of
endogenous hormones. Auxin plays a pivotal role in regulation of plant growth such as embryogenesis, organogenesis,
tropic growth, root architecture, flower and fruit development, tissue and
organ patterning, and vascular development.
Primary/early auxin response genes
of gene families, auxin/indole-3-acetic acid (Aux/IAA), Gretchen
Hagen3 (GH3), small auxin up RNA (SAUR), and auxin response
factor (ARF) have been well
known. In Jatropha,
AUX/IAA, SAUR were associated with sex differentiation. Transcriptome analysis of Jatropha
identifies genes associated with auxin biosynthesis and signaling such as JcAUX1, JcTIR1, JcIAA14, ARFs.
ARFs either activating or repressing the
expression of auxin response genes. In Jatropha JcARF1 acts as a transcriptional repressor and JcARF5 as a transcriptional
activator. Trp-dependent
auxin biosynthesis is the main source of auxin production for formation reproductive
organ and patterning of embryos (wang et al 2015). In this pathway, TAA1/TAR1/TAR2
enzymes produces indole-3-pyruvic acid, which is then converted into IAA by
YUCCA (YUC) flavin-dependent monooxygenases (Novak et al., 2012; Korasick
et al., 2013; Ljung, 2013; Zhao, 2014). These YUCs produces auxin during stamen primordia formation by YUC1 and
YUC4 and late stamen development by YUC2 and YUC6 (Cheng et al., 2006;
Cecchetti et al., 2008). In mature gynoecia, YUC4 and
YUC8 were expressed in the style and YUC2 in carpel valve tissues (Cheng
et al., 2006; Martínez-Fernández et al., 2014). SPARSE INFLORESCENCE1 (YUC-like) mutants resulted in small ears and few
kernels in maize (Gallavotti et al., 2008a). ). TAA1 was associated with
early gynoecial development and
was localized within the medial domain of the gynoecia (Stepanova et al., 2008). Increased expression of ARF10,
16, 17 and 18 caused floral organ loss and abnormal
female fertility in Arabidopsis and less seed set rice (Liu
et al., 2010, (Huang et al., 2016a). PIN-FORMED (PIN1), an auxin efflux carrier protein has
been studied in Arabidopsis for its role in ovule formation. PIN1
directs the auxin flow from base of the gynoecium to the top of the gynoecial
tube (Larsson et al., 2014; Moubayidin and Østergaard, 2014). PIN1 expression is regulated by STY1
and STY2 (Ståldal et al., 2008). BARREN
INFLORESCENCE1 (BIF1)
and BARREN INFLORESCENCE2 (BIF2) genes identified in
maize regulates polar auxin transport via up-regulating ZmPIN1a (Gallavotti et al., 2008b). bif1 and bif2 mutants have
reduced number of spikelets/florets and floral organs, and consequently fewer
kernels in maize (McSteen and Hake, 2001; Barazesh and McSteen, 2008; Skirpan et al., 2009). Ectopic
expression of TAPETUM DETERMINANT1 (TPD1) causes abnormal ovule and seed development via
altering auxin signaling (Huang et al., 2016b,c. The rice gene LAZY1 (LA1), which encodes a novel
grass specific protein, is a negative regulator of polar auxin transport (Li et al., 2007). Spikelets
in la1-ref (La1 orthologue) either are not fully developed or
undergo abortion in the tassel tip.

Cytokinin plays an important role in regulating
the size of shoot apical meristem. ExogenousLess is known about cytokinin’s
involvement in floral development. A recent study of cytokinin overproduction
in floral tissues suggested that the hormone influences the number and
development of flowers through the regulation of floral meristem size (Li et al., 2010b). The transgenic plants overexpressing
cytokinin produced an excess of flower primordia, and the flowers developed
extra organs per-whorl. The mechanism by which cytokinin affects flower size
and organ number is through its regulation of the meristem maintenance
gene CLAVATA 1 (CLV1) and its regulatory effect
on WUSCHEL (WUS) (Lindsay et al., 2006). In addition to cytokinin’s early influence
over flower development, the hormone has also been observed to be involved in
the development of gynoecia. Recent studies have observed that cytokinin plays
a role in gynoecia and fruit morphogenesis and patterning (Marsch-Martinez et al., 2012) as well as having an effect on seed number (Bartrina et al., 2011). Similarly, cytokinins have generally
been found to be feminizing in Mercurialis annua , Vitis vinifera , Spinacia
oleracea , Asparagus offi cinalis , and Solanum carolinense ( Negi and Olmo,
1966 ; Chailakhyan and Khryanin, 1978 ;

Gibberellic
acids (GAs) plays an important role in flower sex differentiation, especially
promoting the development of stamens in monoecious plants. In Jatropha, GIBBERELLIN OXIDASES (GA20OX and GA3OX) were identified and involved in GA
biosynthesis. They promote stamen development and the exogenous GA3 treatment resulted
in arrest of pistil development in females, allowing the male to develop. GA3
and GA4 induced stamens in gynoecia cucumber plants. In other monoecious plants, GA treatment
didn’t develop stamens in gynoecious flowers but it promoted the stamen
development in monoecious female developing bisexual flowers. GID1, a positive
regulator of GA-signaling pathway participates in the embryo sac development in
female flowers whereas GASA4 functions in stamen differentiation in Jatropha. However, in maize plant GAS promotes
carpel development by arresting stamens. ANTHER
EAR1 (AN1) gene is necessary for the formation
of ent-kaurene, a precursor of GA and mutation in this gene results in bisexual
flowers in ears. GA deficiency greatly affects male flowers by causing partial
or complete male sterile plants. Conversely, GA deficiencies promote stamen
maturation in the maize ear and abnormal anther dehiscence and shortened anther
filaments in Arabidopsis (Bensen et al., 1995;
Fujioka et al., 1988;
abnormal anther dehiscence
and shortened anther filaments). Taken together, in both dicots and monocots
the male organ development is sensitive to GA, however, its effects are
opposite.

Jasmonic acid and Brassinosteroids are
involved in flower development as well as stamen, pollen maturation and male
fertility (Stintzi and Browse, 2000; Ishiguro et al., 2001; Park et al., 2002
). Brassinosteroids also results in abortion of
pistils in staminate maize flowers. AG,
a floral organ identity gene controls the stamen maturation through regulation
of a jamonate biosynthesis late developemtal stages in Arabidopsis (Ito et al.,
2007). In addition to its involvement in stamen development, jasmonate has
recently been observed to play a role in the late stages of petal development (
Brioudes et al., 2009 ). Mutants
defective in synthesis or signaling of jasmonates and brassinosteroid signaling results in male
sterility in Arbidopsis, maize and tomato (Li et al., 2004; Ye et al., 2010). constitutive
photomorphogenesis and dwarfism (cpd)
and brassinosteroid-insensitive1 (bri1) mutants produces defective pollens with
limited viability and brassinosteroid-insensitive2 (bin2) mutants are male
sterile (Li et al., 2001). BRs control male
fertility via regulating expression such as SPL/NZZ,
DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION 1 (TDF1), ABORTED MICROSPORES (AMS), and MS1 and MS2 genes critical for anther and pollen
development, (Ye et al., 2010). Thus,
these two hormones promotes the male organs development.

Ethylene promotes female
flowering in cucumber, Arabidopsis and tobacco. CsACO2 (OXIDASE GENE2) encodes an
ACC OXIDASE which oxidizes ethylene intermediates to form ethylene. Transgenic Arabidopsis plants expressing CsACO2 under control of the AP3 promoter display repressed stamen
development resulting in male sterility (Yin and Quinn, 1995; Duan et al., 2008). Down-regulation of the
ethylene receptor gene ETR1 results in the decrease of the
ETR1-interacting kinase CTR1, which a repressor of the ethylene signaling. Loss
of ETR1 fails the formation of ETR1-CTR1 complex. Consequently, de-suppression
of the ethylene response pathway causes the production of female flowers in Arabidopsis (Wang et al., 2010). So far, little is known
about the effects of ethylene on flower development in monocots, including
maize. In tomato, ethylene was observed in
the pistils (Llop-Tous et al., 2000), while in tobacco, ethylene was observed
in the stigma, style, and ovary but not in its pollen and anthers (De Martinis
and Mariani, 1999 ). Expression of ethylene receptors have been observed in the
inflorescence, fl oral meristems, and developing petals and ovules of
Arabidopsis (Sakai et al., 1998). Additionally, ethylene has been implicated in
the expression of floral organ identity genes in tomato. The expression of
TAG-1, an AG orthologue, was suppressed by the ethylene inhibitor 1-MCP
(Bartley and Ishida, 2007). Furthermore, stress-induced ethylene can delay flowering
by repressing GA levels (Achard et al., 2007). As a result, repressor DELLA
proteins accumulate and suppress SOC1 and LFY expression.